Why Carbon monoxide is called silent killer?

Carbon monoxide (CO) is a tasteless, odorless and colorless gas, so it can be drawn into the human body undetected while breathing and it gets absorbed into blood stream. It then form complex compound with iron found in the haemoglobin in blood, upon which haemoglobin cannot act as a carrier of oxygen anymore. As a result, the human dies instantly from being unable to breath and heart disease. This is why CO is called the silent Icier gas.